1. Field of the Invention
The invention relates to endoprostheses of the type which include a stem for insertion into a canal of a bone, such as a medullary canal of a femur.
2. Description of the Prior Art
Endoprostheses include implants which rely on an accurate press-fit to become established between an uncoated stem and the surrounding bone of the femur, implants which rely on tissue ingrowth into a porous soft coating which covers the entire stem, and implants which rely in part on a postoperative biological bond to become established between the surrounding bone and a hard surface porosity on the proximal stem.
Initially, these hard surface porosities were porous coatings bonded to the entire surface of the stem. It was found that bone would bond to the apposite hard porous coating. While such bonding was desired for the proximal stem, it was very detrimental to the distal femur, as subsequently explained.
In an effort to prevent this distal bond, in recent hip joint endoprostheses, the hard surface porosity is confined mostly to the proximal stem, and the distal stem surface is purposely left bare.
Unfortunately, clinical evidence has shown that the various attempts, at using hard porosities for achieving long-lasting, adequate proximal load transfer and proximal implant stability and fixation, have not been entirely satisfactory because a thin bone phase forms externally on the distal femur, and a dense and thick bone forms under the stem tip within the medullary canal.
Such a dense and thick bone under the stem tip will increasingly shunt the load from the proximal femur to the distal femur in the region apposite to the stem tip, and will lead to proximal bone resorption. Femoral bone resorption reduces the proximal femur's resistance to implant rotation and further accelerates the bone resorption process, which is unavoidably accompanied by implant instability, great discomfort, and severe pain.
Even an endoprosthesis having a soft, tissue-promoting coating over its entire stem surface has been found also to experience thin bone growth around the stem tip.
It is an object of this invention to provide different types of endoprostheses with bone resorption preventing means that alleviate the load shunting problem.